A VCSEL is a semiconductor laser consisting of a semiconductor layer of optically active material, such as gallium arsenide or indium gallium arsenide, sandwiched between mirror stacks formed of highly-reflective layers of metallic material, dielectric material, epitaxially-grown semiconductor material, or combinations thereof. Laser structures require optical confinement in an optical cavity and carrier confinement to efficiently convert pumping electrons into stimulated photons through population inversion. The standing wave of reflected electromagnetic energy in the optical cavity has a characteristic cross-section giving rise to an electromagnetic mode. A desirable electromagnetic mode is the single fundamental mode, for example, the HE,.sub.11 mode of a cylindrical waveguide.
Referring to FIG. 1, an exemplary conventional VCSEL is constructed on a GaAs semiconductor substrate 12. A second mirror stack 14 is disposed above the substrate. The second mirror stack is organized as a system of alternating layers of high and low-refractive index materials forming a distributed Bragg reflector (DBR). A gain region 16 including an active element for photon generation is disposed above second mirror stack 14 and a conducting layer 18 is disposed above the gain region. A first mirror stack 20 is located above the conducting layer. The first mirror stack is likewise organized as a system of alternating layers of high and low-refractive index materials forming a distributed Bragg reflector (DBR). The second mirror stack 14, the gain region 16, and the first mirror stack 20 form an optical resonating cavity having a central vertical axis 22.
For light emission in the wavelength range of about 600 to 1000 nanometers, the high-refractive index layers are preferably aluminum arsenide, AlAs, or aluminum gallium arsenide, AlGaAs; and the low-refractive index layers are preferably gallium arsenide, GaAs, or AlGaAs having a lower aluminum content than the high-refractive index layers. Other compound semiconductor compositions may be used for other wavelength ranges.
The mirror stacks in a VCSEL are highly reflective. The reflectivities of the mirror stacks are defined during epitaxial growth of the VCSEL by adjusting the number of periods (i.e., pairs of alternating layers of high-refractive index and low-refractive index materials) in each mirror stack. VCSELs can be either top-emitting or bottom-emitting.
In the top-emitting VCSEL shown in FIG. 1, the second mirror stack preferably has a reflectivity of greater than 99% and the first mirror stack preferably has a reflectivity of about 95-99%. The number of periods in the first mirror stack is less than that of the second mirror stack. The reflectivity of the first mirror stack is reduced to provide light emission from the VCSEL in the vertical direction as shown by arrow 24.
In a bottom-emitting VCSEL, the reflectivity of the first mirror stack is greater than the reflectivity of the second mirror stack.
A metallized contact 26 is applied to the first mirror stack 20 so that the VCSEL can be electrically pumped. Contact 26 has an annular shape and is centered about central vertical axis 22. A metallized contact 27 is applied to substrate 12, or to the second mirror stack, so that current flows through the first mirror stack, the gain region, and the second mirror stack. An annular current confinement region 28 of high electrical resistivity is located between second mirror 14 and conducting layer 18, circumscribing the gain region, and centered about central vertical axis 22. The high-resistivity current confinement region 28 channels injected current into a central portion of active region 16 for more efficient light generation therein.
Presently, commercially practical VCSELs are limited in the wavelengths of laser light they can produce. The wavelength of the laser light is determined by the active material used in the gain region of the laser. A given active material has a characteristic wavelength or wavelengths at which it will lase based on the atomic structure of the material. Of commonly used commercial materials for commercially practical VCSELs, gallium arsenide and indium gallium arsenide both can produce light with a wavelength in the infrared portion of the light spectrum. Aluminum gallium arsenide, indium aluminum gallium arsenide and indium gallium arsenide phosphorous produce light with a wavelength in the visible red portion of the light spectrum.
Because the wavelength of a laser beam determines in part how tightly the laser beam can be focused, it is often desired to produce laser light at wavelengths for which lasing materials do not exist. More specifically, it is desired to shorten the wavelength of light produced using a given active material.
A known technique for obtaining laser energy having a shorter wavelength than that characteristically produced is to utilize the non-linear interaction between light and some forms of matter to generate harmonics at multiples of the characteristic wavelength. In commercial, non-integrated lasers, a non-linear crystal is commonly employed for such non-linear interaction. As a laser beam passes through the crystal, non-linear interactions between that laser beam and the crystal generate an electromagnetic wave at half the wavelength of the laser beam. In these non-integrated lasers, the magnitude of the non-linear effect is proportional to the square of the optical power of the laser. As a result, high powered lasers which make wavelength shortening practical are needed.